1. Field of the Invention
The invention relates to a neutron shielding arrangement for a nuclear reactor, and more particularly to an arrangement employing a neutron shield panel to shield a selected region of a nuclear reactor vessel against the impingement of high energy neutrons emanating from the nuclear reactor core.
2. Description of the Prior Art
In pressurized water-moderated and boiling water nuclear reactors, both of which are used for producing steam to drive a steam turbine, a nuclear reactor core and other control apparatus, commonly referred to as reactor internals, are disposed in a metal vessel filled with water. Fission takes place within the reactor core whereby the energy of the fission products is transmitted to the water. In a pressurized water-moderated reactor the heated water is pumped from the reactor vessel through a heat exchanger in which it transfers its heat energy to another circuit of water to form steam to drive a turbine. In the boiling water reactor the energy of the fission products is transmitted to the water in the reactor vessel to form steam to drive a turbine. In both types of reactor it has been found that neutron flux imposed on the vessel from the reactor core results in the material of the vessel becoming embrittled. That is, with prolonged exposure to high energy neutrons the fracture toughness of the vessel is reduced and could ultimately result in fracturing of the vessel should the condition be allowed to continue.
Several design techniques have been utilized in the past to reduce the radiation exposure of the vessel. For example, in pressurized water reactors manufacturers have employed a steel thermal shield to reduce the radiation exposure of the vessel. In one such design the core-barrel, which surrounds and supports the reactor core, it itself surrounded by a cylindrical steel wall which is 2 to 3 inches thick and which is situated approximately an equal distance from the inner surface of the pressure vessel and the outer surface of the core-barrel. In an alternative design disclosed in U.S. Pat. No. 3,868,302, recognizing that the neutron flux level impinging on the inner surface of a reactor vessel varies markedly in the circumferential direction, the thickness of the core-barrel is selectively increased in those regions where a high flux exists. If the thickness of the core-barrel is increased in the high flux regions by an amount corresponding to the thickness of the cylindrical thermal shield first mentioned above, the maximum radiation exposure of the reactor vessel is essentially the same as the level that would result with a separate cylindrical thermal shield.
The foregoing thermal shield designs satisfy standards for allowable fluence (the number of neutrons impinging upon a unit area) over the operating life of certain nuclear reactors. However, it has been found that the chemical composition of the reactor vessel in some nuclear reactors causes the vessel to have a high rate of embrittlement even with such shielding. Specifically, it has been found that when the steel forgings or plates which comprise the vessel contain significant residual amounts of copper and nickel (approximately 1% combined), the rate of embrittlement of the vessel is significantly increased over vessels which do not contain such impurities. Additionally, any welds which join the steel forgings or plates together to form the vessel and which contain significant residual amounts of copper and nickel are also subject to increased embrittlement rates.
The known thermal shielding designs discussed above may not have sufficient neutron fluence reduction capability with respect to vessels and welds therein which contain significant residual amounts of copper and nickel to prevent the steel of such vessels and/or the welds therein from exceeding proposed Nuclear Regulatory Commission screening criteria regarding allowable embrittlement within the projected operating life of the reactor. The known thermal shield designs can reduce the fluence by approximately 20% to 40%, depending on the particular shield and nuclear reactor. It has been determined, however, that vessels and/or welds in such vessels containing significant residual amounts of copper and nickel and located in the high fluence regions of the reactor may require shielding which reduces the fluence by a factor of 4.0 or more.
Fuel mnagement techniques are known by which the fuel assemblies within the core can be arranged in a manner to minimize the neutrons flowing out of the core. Fuel management techniques have been employed to reduce the fluence by as much as a factor of 2. However, the combination of fuel management techniques and known shielding designs still may not reduce the fluence enough to keep the rate of embrittlement of a reactor vessel containing significant residual amounts of copper and nickel within acceptable limits.